COMMENTS

The northern Marche coastal zone is presently affected only by low seismic 
activity, but damaging earthquakes struck this area in the recent past (e.g. in 
1930, Senigallia). A NW-SE trending minimum stress axis and a NE-SW 
trending maximum compressional stress axis were shown respectively by 
bore-hole breakouts and earthquake focal mechanisms (Mariucci et al., 1999; 
Frepoli and Amato, 1997). The available field geological observations are still 
inadequate to devise a fully convincing correlation between the occurrence of 
earthquakes and realistic seismogenic sources, not only in the area of 
Senigallia but also along most of the northern Adriatic coastal belt.

Much of the tectonic activity in this region is contended between the so called 
"transverse structures", i.e. NE-SW-trending, sub-vertical faults characterised 
by relatively unclear kinematics, and NW-SE trending, SW-dipping thrust-
faults driving the growth of gentle buckle-folds. The activity of the transverse 
structures that appear in many studies is questioned by Elmi et al. (1991), who 
invoke the misleading effects of lithological factors and exposure of rocks to 
solar radiation in interpreting the response of surface processes to tectonics. 
Elmi et al. (1991) also hint at the possibility of a general tilting of this region 
around a NW-SE axis testified by the convergence of alluvial terrace treads in 
the lower reaches of the main rivers. The presence of growing anticlines in 
the coastal zone is proposed by Nanni and Vivalda (1987). Later studies 
neither clearly dispute this information nor substantially endorse it. 
However, the most realistic hypothesis that can be envisaged to-date on the 
basis of the local geologic setting considers the blind thrust-faults located at the 
leading-edge of the Apennine accretionary prism as the main active and 
probably seismogenic faults in the area. In addition, it seems likely that two 
parallel alignments of this sort of faults exist along the northern Marche 
offshore. The "transverse structures" may thus represent tear faults that 
accommodate strain between different thrust-fault segments.

The seismogenic source of the 30 October 1930 Senigallia earthquake may be 
correlated with a blind thrust-fault driving the growth of the coastal anticline 
and detected by Bally et al. (1986) through geophysical prospecting. The 
occurrence of a sizeable tsunami (Boschi et al., 1995) constraints the fault to be 
along or very near to the coast. The geometry and width of fault at depth are 
based on a seismic reflection profile by Bally et al. (1986). Its length is 
constrained on the basis of major fluctuations of the fold axial-plane. This set 
of hypotheses requires a seismogenic fault located a few km to the NW of the 
source inferred from historical data. This difference in the location of the 
geology- and intensity-based sources could be due to anomalously high 
intensity data to the SE of the earthquake epicentre (Molin and Mucci, 1990). 
However, both solutions strike nearly  the same. It is worth noting that 
Renner and Slejko (1994) locate the 1930 earthquake just off the coast of 
Senigallia (13.186 E; 43.775 N). This solution requires a source some 15 km 
to the north of the source computed on the basis of intensity data. In this last 
case the earthquake fault should be located offshore and would represent an 
alternative hypothesis to the source proposed here. One can compare the two 
confronting hypotheses in the figure "Seismic reflection profiles by Bally et al., 
1986".


OPEN QUESTIONS

1) What is the most reliable seismic source for the 1930 earthquake and, in 
more general terms, for large earthquakes in the northern Adriatic coastal 
belt?

2) To what type of geologic structures may these sources be correlated?

3) Are the coastal anticline presently growing?

4) What are the kinematics of the "transverse" faults? Could these faults be 
also seismogenic?
